Additive/additive interactions in boundary lubrication—a study of film formation and tenacity

Abstract

In this work the interactions between two key additives in current lubricants (ZDDP and MoDTC) and the effect of these interactions on tribofilm formation and evolution under boundary lubrication are studied. Tribofilms are formed using a pin-on-reciprocating plate tribometer and the oils comprise combinations of base oil (PAO6), zinc dialkyldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) additives. The tribological and chemical characteristics of tribofilms are probed using measurement of friction and film characteristics. Wear films have been chemically characterised using Energy Dispersive X-Ray analysis (EDX) and X-Ray Photoelectron Spectroscopy (XPS). In order to investigate the morphology of the reaction films formed, Atomic Force Microscopy (AFM) was used. A new testing procedure is applied to specifically investigate how the stability of the tribofilm is affected by changing the oil additive package. This testing has given new information on ZDDP and MoDTC interactions in tribofilm formation and on the tenacity of the tribofilms. The surface analyses have shown that by-products of MoDTC decomposition form a tribofilm in the wear scar leading to the formation of MoS2 and consequently friction reduction.